Prolactinomia. I have it after I lost a baby. My hormones just never went back to normal.
I have had it for at least 10 years. I take medicine daily to decrease the hormones so I can have a regular cycle. I do not take the meds when pregnant or nursing. I see an endocrinologist for this. I have a MRI every year and they do not see a tumor but the blood work shows high prolactin levels. You would be amazed at how many people have this (even men).
I have found that with diet, exercise and a great outlook I can greatly reduce my dependency to meds.
Have your sister see an endocrinologist referred by her OB/GYN. She needs to get this checked out as soon as possible. I wish her luck - she will be alright.